Among others, the missing radvd dependency showed up as:

error: Failed to start network ipv6net
error: Cannot find radvd - Possibly the package isn't installed: No such file
or directory

even when radvd was installed, because the RADVD preprocessor
symbol was missing at configure time.

* libvirt.spec.in (with_network): Add BuildRequires for radvd,
iptables, and ip6tables.
(BuildRequires): Add libxslt and augeas for docs and test.
(with_libvirtd): Add module-init-tools for modprobe.
(with_nwfilter): Add BuildRequires for ebtables.
(with_esx): Fix esx build on RHEL 5, thanks to curl-devel rename.